I just came up with an idea. Wouldn't it be possible to tilt them back somewhat to become less off-axis?
Just place some spacers or something underneath the front of the cabinet to tilt them back a little.

I think i may go calculate how much i'd need to tilt them. But not today, been doing enough math for today :P

So, it is well worth the extra cost for the el70 then I suppose. Thanks!

If you wanted to stick with MA drivers, the Alpair 6P would be another candidate - the 6M's can dig deep enough for HT rear/surrounds, and I've heard the 6P solo in a smallish box - I can't see either of them not working well as mid-tweeter. The big question would be choice and method of XO (i.e. frequency point and active/passive), and amplification (which could also be DIY'ed) .

A school project with thorough documentation of the decision making and design process, as well as the construction, could be very impressive to the teachers, and a great shared learning experience. You might even be able to sell a few.